1. Field of the Invention
The present invention relates to a spring wire rod. More particularly, the present invention relates to a spring wire rod to be made into valve springs, clutch springs, suspension springs, etc. with improved fatigue characteristics.
2. Description of the Related Art
It is known that any spring steel containing hard non-metallic inclusions is subject to breakage triggered by them. One way proposed so far to improve the fatigue characteristics of spring steel, particularly silicon killed steel, is by conversion of hard inclusions into those having a lower melting point. U.S. Pat. No. 6,328,820, for example, teaches that steel improves in fatigue characteristics if oxide inclusions therein have a controlled composition (SiO2: 35-75 wt %, Al2O3: 5-30 wt %, CaO: 10-50 wt %, MgO: 5 wt % or less), which lowers the melting point below 1400° C., and a reduced thickness.
Aluminum killed steel, however, is not studied so deeply as silicon killed steel. A common measure employed for aluminum killed steel is the reduction of oxygen content in steel which leads to fine oxide inclusions. Japanese Patent Laid-open No. 2005-2441 discloses a method for reducing the average particle diameter of inclusions (sulfides, nitrides, and compounds thereof) below 7 μm in order to improve the resistance of notch fatigue characteristics of aluminum killed steel.